Ruby 1.9.3-p327 was released to correct a hash-flooding DoS vulnerability that only affects 1.9.x and the 2.0.0 preview [1].
As noted in the upstream report:
Carefully crafted sequence of strings can cause a denial of service attack on the service that parses the sequence to create a Hash object by using the strings as keys. For instance, this vulnerability affects web application that parses the JSON data sent from untrusted entity.
This vulnerability is similar to CVS-2011-4815 for ruby 1.8.7. ruby 1.9 versions were using modified MurmurHash function but it's reported that there is a way to create sequence of strings that collide their hash values each other. This fix changes the Hash function of String object from the MurmurHash to SipHash 2-4.
Ruby 1.8.x is not noted as being affected by this flaw.
[1] http://www.ruby-lang.org/en/news/2012/11/09/ruby19-hashdos-cve-2012-5371/